Detonation energy of commercial explosives can be broadly divided into two forms—shock energy and heave energy. Shock energy fractures and fragments rock. Heave energy moves blasted rock after fracture and fragmentation. In general the higher the velocity of detonation (VOD) of an explosive the higher proportion of shock energy the explosive is likely to exhibit.
Certain mining applications require the use of explosives that exhibit a combination of low shock energy and high heave energy. This allows fragmentation to be controlled (high shock energy produces significant amounts of dust sized fines) and in turn reduces excavation costs. In softer rock and coal mining applications, for example, the use of explosives that provide a relatively high proportion of heave energy can lead to significant savings downstream for the mine operation because collection of blasted rock then becomes easier. In quarry applications, fragmentation control and reduction of fines is also very attractive.
Current commercial explosives offer a range of shock and heave energies. For example, ANFO (ammonium nitrate/fuel oil) tends to provide a particular balance between shock and heave energies (low shock energy and high heave energy), and is frequently used as a reference point for assessing blast performance. In fact, ANFO with all of its ammonium nitrate present as prill exhibits what is conventionally believed to be an excellent combination of shock (fragmentation) and heave properties for many rock blasting and collection situations.
In contrast, homogeneous fluid explosive compositions, such as ammonium nitrate emulsion explosives tend to provide high shock energy and low heave energy. It is well known that such emulsion explosives tend to have relatively high velocities of detonation and correspondingly high pressure in the chemical reaction zone. This results in a high shock explosive that is well suited to fragmenting rock, but that has relatively low heave energy to move fragmented rock. Various water gel explosives provide a similar range of performance to emulsion explosives.
In practice, materials that modify explosive characteristics, such as ammonium nitrate (AN) prill are conventionally added to emulsion explosives to enhance their overall heave properties. Prills are understood to contribute to a late burn in the post detonation zone and this manifests itself as heave energy rather than shock energy.
The explosive properties of prill-containing explosive compositions are closely related to the explosive characteristics of the prill itself and, in turn, the explosive characteristics are influenced by factors including the physical features, internal structures and chemical composition of the prill. However, such factors may vary within a wide range depending on such things as the manufacturing technology used to produce the prill, the type and/or content of additives (and/or contaminants) present in the prill, the manner in which the prill is stored and/or transported, and the context of use of the explosive, including the degree of confinement and environmental factors, such as temperature and humidity. As a result, the detonation performance (including the energy release characteristics) of conventional prill-containing explosives tends to be highly variable. Explosive formulations with a high concentration of prill are also very difficult to pump into a blasthole. In contrast, emulsion explosives and slurry formulations are readily pumped and particularly useful in wet conditions. ANFO based formulations can only be used in wet conditions after dewatering of the boreholes.
A further consideration in relation to the use of ANFO and AN prill-containing emulsion explosives is the cost of manufacture of AN prill. AN prill manufacturing towers represent a significant fraction of capital expenditure associated with an ammonium nitrate production facility. Prilling is also a highly energy intensive process that adds significantly to the carbon footprint associated with these type of explosives.
Against this background, the Applicant has devised an explosive for commercial blasting operations that does not require the use of ammonium nitrate prill and that therefore does not suffer the potential problems associated with the use of prill, but that can achieve at least comparable rock blasting performance as currently used ANFO and AN prill-containing explosives. The explosive composition devised by the Applicant exhibits the desirable features of conventional ANFO and AN prill-containing explosives in terms of detonation energy ratio as between shock and heave energies, but that is free of the practical (and economic) constraints associated with the use of such prill-containing conventional explosives.
More specifically, the Applicant has devised an explosive composition comprising a liquid energetic material and sensitizing voids, wherein the sensitizing voids are present in the liquid energetic material with a non-random distribution, and wherein the liquid energetic material comprises (a) regions in which the sensitizing voids are sufficiently concentrated to render those regions detonable and (b) regions in which the sensitizing voids are not so concentrated. The explosive composition is therefore defined with reference to its internal structure. Explosive compositions that have this particular internal structure/void distribution exhibit desirable features of conventional ANFO and AN prill-containing explosives in terms of detonation energy ratio as between shock and heave energies, but that is free of the practical (and economic) constraints associated with the use of such prill-containing conventional explosives. For ease of reference the explosive compositions that may be produced in accordance with the present invention are referred to in general terms as having a non-random distribution of sensitizing voids in a liquid energetic material. Such explosive compositions are described in the Applicant's International patent application nos. PCT/AU2012/001527 and PCT/AU2012/001528, the contents of which are incorporated herein by reference. The invention may have particular applicability to such explosive compositions. The contents of the Applicant's International patent application nos. PCT/AU2012/001527 and PCT/AU2012/001528 are set out in detail.
Moreover, with explosive compositions that have a non-random void distribution, blast performance/characteristics can be adjusted in order to suit an array of different blasting requirements. For example, it may be desired to vary explosive performance across a blast field by loading individual blastholes with an explosive formulation that is most well suited to the characteristics of each blasthole, the prevailing geological conditions and/or the intended blast outcome. Conventional blasting practice has generally been to deliver the same explosive formulation to each blasthole in a blast field irrespective of blasthole characteristics etc. This approach can yield acceptable results but there is scope for improvement by designing or matching the explosive formulation used on a hole-by-hole basis. However, this brings with it certain practical challenges, not least how to undertake formulation manufacture, formulation variation and blasthole loading in a manner that is convenient and economical to implement. The present invention seeks to provide solutions that meet these practical challenges.